Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnly/idempotent/openWorld, and the description still adds material context: the response fields (rate, published rate date, provider_code), the retrieved_at-vs-rate-date distinction, and the paid-access requirement ($0.0010 USDC via x402 on Base mainnet). The payment/auth trait is exactly the kind of behavior an agent cannot infer from structured fields.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Every clause carries distinct information and the critical disambiguation (reference rate, not converted amount) is front-loaded. The single dense paragraph is slightly over-packed, but nothing is redundant with the schema or annotations.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a two-parameter read tool with an output schema, the remaining burden is disambiguation, provenance, freshness, and cost — all of which are covered. Nothing an agent needs in order to call it correctly is missing.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema description coverage is 0%, so the description must carry parameter meaning — it does, by specifying three-letter ISO codes restricted to ECB-supported currencies, plus a concrete example object. It stops short of naming the actual supported currency set or case requirements, which the schema pattern handles only partially.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

"Get a reference exchange rate, not a converted amount" states a specific verb and resource and immediately fences off the nearest sibling, convert_currency. The ECB/Frankfurter sourcing narrows the domain further, so an agent can pick this over get_historical_fx_rate or convert_currency without opening a schema.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

It gives explicit when-to-use context: reference rate rather than conversion, latest available daily data rather than a real-time trading feed or executable quote. It does not explicitly name get_historical_fx_rate as the alternative for past dates, so routing to that sibling is left to inference from "latest available".

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.